Cement kiln operations are significantly influenced by operational parameters that vary with changing environmental factors, including ambient temperature, humidity, coal composition, etc. Gas temperatures in the kiln, including furnace exit gas temperatures, are influenced by these factors, as well as by adjustments that can be made to the furnace apparatus, such as burner configuration and orientation, air flow rate, fuel feed rate, etc.
Gas temperatures profoundly affect the performance of a kiln in several ways. The thermal NOx formation rate increases exponentially at temperatures over 2700° F. There is strong regulatory pressure to reduce NOx emissions, but the fundamental knowledge of furnace exit gas temperatures, the primary factor in NOx formation, is lacking in cement kilns because existing temperature measurement technology is incapable of producing accurate temperature data in large boilers.
The long felt need for improved accurate temperature measurement in petro-chemical plants and cement kilns exists because the prior art measurement techniques are inadequate to reliably produce accurate temperature measurement in these environments. Thermocouple probes are unreliable and fail quickly in corrosive environments. Optical methods have limited penetration and are difficult to interpret. Prior acoustic methods have not proven accurately in noisy and dirty environments, in part because they are unable to accurately detect the onset of the acoustic signal in high amplitude background noise.
Thus, there has long been a need for accurate temperature measurements in cement kilns that enable improvements to be made in efficiency, and also in product uniformity and consistency. The temperature measurement would also be useful in minimizing NOx formation by reducing the dwell time at high temperature.
An acoustic pyrometer is shown in U.S. Pat. Nos. 6,386,755; 6,726,358, and 6,834,992. The acoustic pyrometer shown in these patents has been proven to be accurate and reliable in large boilers, but has required higher air pressure for its operation than is typically available in plant air systems, so an air amplifier is normally used to boost the air pressure to the desired range for optimal operation. The air amplifier is an additional expense and requires additional maintenance, and would be a candidate for elimination from the acoustic pyrometer system if a satisfactory signal generator could be designed that did not require elevated air pressure for operation. Thus, a need exists for an acoustic pyrometer that can operate accurately and reliably with a source of air pressure afforded by typical plant air systems.